1. Field of the Invention
This invention relates generally to state of charge determination of cells in a battery pack and, more particularly, to a method for modeling changes in the state of charge vs. open circuit voltage curve for battery cells as the cells age, where the curve is modeled using parameter regression or optimization in a physics-based model, and the resulting cell performance curve is used to improve state of charge determination and capacity estimation.
2. Discussion of the Related Art
Electric vehicles and gasoline-electric or diesel-electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid-electric vehicles offer several desirable features, such as reducing or eliminating emissions and petroleum-based fuel consumption at the consumer level, and potentially lower operating costs. A key subsystem of electric and hybrid-electric vehicles is the battery pack, which can represent a substantial proportion of the vehicle's cost. Battery packs in these vehicles typically consist of numerous interconnected cells, which are able to deliver a lot of power on demand. Maximizing battery pack performance and life are key considerations in the design and operation of electric and hybrid electric vehicles.
A typical electric vehicle battery pack includes two or more battery pack sections, with each section containing many individual battery cells as needed to provide the required voltage and capacity. In order to optimize the performance and durability of the battery pack, it is important to monitor the state of charge of the cells. State of charge of a cell is typically determined based on the open circuit voltage of the cell, using a known relationship which is defined in the form of a state of charge vs. open circuit voltage (SOC-OCV) curve. However, as battery cells age, experiencing repeated charge-discharge cycles, the relationship between open circuit voltage and state of charge changes. While it is possible to disregard the change in state of charge as a function of open circuit voltage in aging battery cells, for example by using a conservatively low estimate of state of charge during vehicle operation, it is far preferable to accurately determine state of charge of battery cells even as they age. The accurate determination of state of charge as a function of open circuit voltage is important both during charging of the battery pack and during discharging as the vehicle is driven.
Various methods of characterizing performance changes in aging battery cells are known in the art. Many of these methods are empirically-based; that is, they predict changes in the battery cell's performance based on the number of charge-discharge cycles, using data from experimental measurements. Others of these methods simply estimate capacity fade, or reduction in energy storage capacity over time, but do not attempt to characterize the changes in the SOC-OCV curve. However, none of the known methods uses in-vehicle measurements and a physics-based model to determine the actual physical state of the battery pack's cells, including the SOC-OCV curve, as the battery pack ages.